Printed circuit boards have long been used in the fabrication of electronic components. Printed circuitry provides a support for discrete components, while providing most of the electrical interconnections between components. The circuitry pattern may be transferred to the board by photographic or lithographic techniques, thus permitting mass production without labor-intensive soldering. Circuit boards are generally manufactured from epoxy resin (typically containing reinforcing fiberglass fibers) clad on one or both sides with copper foil. The circuit pattern is applied to one or both sides marking the pathways with a resist, etching the non-masked copper from the board, and removing the resist.
Single and double sided boards are inherently limited with regard to the density of components they can support and the amount of current they can carry. Printed circuit paths cannot cross, thus requiring careful arrangement of components which may be suboptimum where components may interfere electromagnetically or generate excessive heat. The solution now used is the multilayer circuit board. Several single or double sided circuit boards are prepared, and are laminated together (separated by an appropriate dielectric, typically partially-cured resin) under pressure and heat to form an integral composite board having several layer of circuitry embedded within.
The circuit paths are generally applied to the boards using a photoresist technique. A photoresist material is applied either as a liquid, or more commonly, as a film. The treated board is then exposed to uv light through a negative image of the circuit pattern. The radiation cures the photoresist, causing it to harden. Then, the unexposed portions of the resist are removed, typically with an alkaline solution, and the exposed copper is etched using ammoniacal copper chloride, cupric chloride, acid or ferric chloride. The hardened photoresist prevents copper beneath the circuit pattern from etching. The hardened resist is then removed, leaving the circuit pattern in copper.
Modern circuit boards are designed with circuit paths as narrow as 0.1 mm, with little separation between adjacent paths. Thus, it is critical that the photoresist remain in place on the copper-clad board when applied. Any displacement or lifting of the photoresist during processing may cause inaccuracies in the circuit pattern, and may cause mis-registration from layer to layer of multi-layer board, as the layers are connected by communicating holes. To insure adhesion between the copper surface and the photoresist resin, the metal surface is generally roughened. Where the metal layer and its supporting board are sufficiently sturdy, this may be accomplished by mechanical means, for example, by scrubbing with a wire brush. However, the boards currently used for internal layers of multilayer circuit boards are generally too thin to withstand such vigorous mechanical treatment. The conventional procedure calls for etching the copper surface with acid peroxide blends or ferric chloride to provide a rough surface.
Another metal-resin adhesion problem occurs between the finished circuit paths and the solder mask. In this instance, etching the circuit paths may leave the board with a dull appearance. As the solder mask is applied to the outer, visible surface of the board, a dull appearance is detrimental.